1. Field of the Invention
This invention relates to a curable polymeric composition and the use of the composition in protecting a substrate, in particular a metallic pipe.
2. Introduction to the Invention
To protect substrates such as metallic pipes, pipe joints, and pipelines from corrosion, mechanical damage resulting from abrasion, or degradation due to exposure to sun, soil, moisture, or other elements, a protective coating may be applied to the exterior of the pipe. The protective layer is often polymeric and may be in the form of a polymer tape which is coated with a mastic layer and is then applied to the pipe, a polymer layer which is sprayed or painted onto the pipe, or a fusion bonded epoxy coating which has been applied in the factory. Alternatively, the protective layer may be in form of a heat-recoverable article, i.e. a heat-shrinkable polymeric article such as a sleeve, sheet, or tape, which is recovered onto a pipe or substrate. Such heat-recoverable articles are particularly useful in providing protection to pipe welds or joints which are created when pipe is repaired or retrofitted, e.g. when two pieces of pipe are attached to one another. Using a heat-recoverable article allows protection to be applied in the field without the need for difficult, time-consuming, and craft-sensitive procedures.
In order to enhance the adhesion between the pipe and the heat-recoverable article, it is common to use a heat-activatable sealant or a mastic which can bond to both the pipe and the heat-recoverable article. The sealant or mastic is often in the form of a layer which is applied to the surface of the article which is in contact with the substrate. When heat is applied to the article, generally by means of a torch or other heat source, the heat-activatable sealant is activated and the article recovers. A strong bond is thus formed between the polymeric article and the substrate. Alternately or in addition to the heat-activatable sealant, a primer layer, often in the form of a curable polymeric composition such as an epoxy, may be applied to the substrate to improve the adhesion of the heat-recoverable layer to the substrate.
Curable epoxy primers for pipe coating are known. U.S. Pat. No. 3,876,606 (Kehr), the disclosure of which is incorporated herein by reference, discloses a thermosetting epoxy resin which comprises a polyglycidyl ether of polyhydric phenol, a dihydrazide, and a filler such as barium sulfate, calcium carbonate, or mica. The coating has good adhesion to the substrate when subjected to boiling water and also exhibits good cathodic disbanding performance at room temperature and at 60.degree. C. However, the coating is in the form of a powder which is applied to a preheated substrate. Uniform application of such a powder onto a pipe or a pipe joint in the field is difficult, requiring complex equipment and relatively high temperatures.
U.S. Pat. Nos. 4,732,632 and 4,997,685 (Pieslak et al), the disclosures of which are incorporated herein by reference, disclose a method of applying a protective coating onto a substrate. The method comprises applying to the substrate a curable polymeric composition which is a liquid at about 20.degree. C., comprises a resin component and a curing agent, and cures to a substantial extent within 24 hours at a temperature of not more than about 80.degree. C. A polymeric covering layer having an innermost layer of a heat-activatable sealant is applied over the curable composition and the curable composition is then cured while in intimate contact with the innermost layer. The resulting covering has good adhesion to both the substrate and the heat-recoverable article and exhibits good cathodic disbanding performance. It is necessary, however, that the curable composition be selected to meet the specific requirements for each application; a single composition cannot be used for a wide variety of temperature applications.